The present invention relates to seismic exploration of the structure of the Earth Surrounding liquid-containing wells, and more particularly to the type of exploration wherein powerful acoustical impulses are produced by a sound source lowered into the liquid in a well for causing acoustical impulses to penetrate the wall of the well adjacent to the sound source and to travel through the surrounding structure to geophones for recording and analyzing the resulting electrical signals produced by the respective geophones in response to the acoustical impulses reaching them after propogation through the Earth's structure.
A problem which is encountered in such seismic exploration results from the fact that the liquid-filled well bore is a good conductor of tube waves with the inner surfaces of the well wall acting as a wave guide. Thus, relatively strong tube waves travel along the well bore downwardly and upwardly from the sound source each time that it is fired. When these tube waves traveling down along the well bore reach the bottom of the well, or a discontinuity in the liquid-filled well bore, they are strongly reflected, but a significant amount of the energy in these tube waves becomes converted into compressional body waves propogating out from the discontinuities in the well through the surrounding structure of the Earth. Similarly, when these tube waves traveling up along the liquid-filled bore reach the top surface of the liquid, they are strongly reflected, but a significant amount of their energy becomes converted into compressional body waves propagating out from the top of the liquid column through the surrounding structure of the Earth.
These compressional body waves, i.e., sound (accoustical) waves, propagated outwardly through the Earth's structure from the bottom and top of the liquid column and other discontinuities within the well bore, may be called "secondary sound (or acoustical) waves", and they also reach the geophones but at a later time than did the compressional body waves travelling directly from the impulse energy (sound) source. Shear waves travelling directly from the impulsive energy source are propagated more slowly than compressional body waves, about half as fast, and thus the secondary sound waves cause more interference with the shear waves than with the compressional body waves. These secondary sound waves cause the records of signals from the geophones to be cluttered and confusing with unwanted signals which are attempted to be removed during processing of the records before their final graphic display.
The tube waves which travel downwardly or upwardly from the sound source along the liquid-filled well bore are being guided along the side wall of the well bore, and the particle motions of these tube waves occur in both the well liquid and the adjacent solid material of the casing or rock and are in the nature of "Stoneley Waves".